1. Field of the Invention
The present invention relates generally to a process for delivering highly concentrated solar radiation to a material surface to evaporate the material so that it condenses into caged molecules.
More specifically, the invention is directed to a method for producing fullerenes by: providing a solar furnace having a focal point wherein the solar furnace concentrates sunlight; providing a reflective secondary concentrator having an entrance aperture and an exit aperture at the focal point of the solar furnace; providing graphite at the exit aperture of the secondary concentrator; flowing argon gas over the graphite to keep the secondary concentrator free from vaporized carbon; and impinging the concentrated sunlight from the secondary concentrator onto the graphite to vaporize the graphite into a soot containing high amounts of fullerenes.
2. Description of the Prior Art
A short while ago, carbon was considered to exist in a limited number of forms such as diamond, graphite, glassy carbon, amorphous carbon, and a number of high-temperature species that existed in the vapor phase above 2,000.degree. C. However, in 1984, mass spectrometry experiments revealed that carbon could possibly exist in a number of other forms ranging from C.sub.30 to C.sub.190.
Approximately one year subsequent to 1984, the unique stability of molecular allotropic forms such as C.sub.60 and C.sub.70 was demonstrated ([H. W. Kroto et al.; Nature 318, 162 (1985)]). These events led to the discovery of a whole new set of carbon-based substances known as fullerenes. Fullerenes are composed of closed polyhedra or tubes produced by carbon atoms linking together to form hexagons and pentagons as shown in FIG. 1.
The configuration of carbon atoms in fullerenes provides properties that have captured the interest of chemists, physicists, materials scientists, and medical researchers, as fullerenes have been shown to crystallize to form interesting solids and to polymerize in several ways to form new polymers. Also, metal atoms can be placed inside the fullerene cage to form encapsulated systems (i.e. UC.sub.28, LaC.sub.82, etc.), or outside the cage to form catalysts.
The fullerene cage can be reacted with other substances in a number of ways to form new molecules of interest.
Tubules of fullerenes have caught increasing interest as fibers, nanowires, and encapsulants. Fullerenes may also be doped to form electronic materials or reacted to form superconductors.
All of these applications have been discovered since the first macroscopic amounts of the most common fullerene, C.sub.60, were isolated in 1990 [Kratschmer, et al., Nature 347, 354 (1990)].
Much of the work on fullerenes was performed using small amounts of material since sythetic approaches to these forms of carbon yielded limited quantities of material. The major drawback to the commercialization of some of the applicatins mentioned is due to the lack of a large-scaled method for producing and isolating fullerenes.
Synthetic production of fullerenes was first provided using vaporization of grapite in an expanding helium atmosphere [H. W. Kroto, et al., Nature 318, 162 (1985)]. In this method, a Q-switched Nd: YAG laser is focused onto a rotating disc of graphite, whereupon carbon is evaporated or ablated into a high density helium flow. Clusters of soot form and are detected using a time-of-flight mass spectrometer. However, this method of production is sufficient to form only a few micrograms of fullerenes per day, which is only enough for certain limited research purposes.
A more useful method of synthesizing fullerene containing soot is the contact-arc method [Kratschmer, et al., Nature 347, 354 (1990)]. In this method, lightly contacting graphite electrodes are heated electrically by an alternating-current arc welder in an atmosphere of helium at a pressure of about 100 to about 200 torr. the graphite heats to evaporation at the contact and produces soot containing fullerenes. The soot condenses upon cool walls of a chamber, and is scraped off after a run that consumes the electrodes. Fullerenes are extracted from the soot by a solvent, such as tolunene or benzene. This method is capable of producing a few tens of milligrams of fullerenes per run. While the apparatus used can be run in parallel so that the process is capable of producing several grams of fullerenes per day, the process is encumbered by scaling problems. For example, as the diameter of the rods are increased and the current supplied to the rods is increased to increase the amount of graphite evaporated per unit time, the yield of fullerenes decreases.
Rods that are 1/8" in diameter are capable of producing maximum yields of about 30%, while rods that are 1/4" in diameter are capable of producing yields of around 15%, and rods that are 1/2" in diameter only have yields that are no more than 7%. The linear decrease in yield with an increase in rod diameter is not understood, but a reasonable conjecture put forth by Chibante, et al. [(J. Phys. Chem. 97(34), 8696 (1993)], is that the intense ultraviolet light in the plasma region of the arc may destroy fullerenes before they can exit that region.
Howard, et al. in Nature 352,139 (1991) discloses a third method of producing fullerenes. This method entails burning hydrocarbon feeds in an oxygen deficient flame or sooty flame. Benzene is used as a hydrocarbon source, with an argon diluted oxygen supply. In this method, it was found that soot yields are 0.2 to 12% of the carbon feed, and this gives a maximum yield of fullerenes of 0.3% of the carbon feed. This synthesis process is too costly to compete with the contact-arc process.
U.S. Pat. No. 4,874,596 discloses a method of changing the structure of a solid material in the form of carbon by converting the carbon to one or more other forms including diamond by the intense heat and shock wave force generated and transmitted through the material by intense radiation. This process does not provide caged molecular forms (fullerenes) nor does it encroach upon the physical concepts required for their production.
The three prior art methods of producing fullerenes, namely, laser ablation of graphite targets, the carbon arc process (also called the contact-arc process) and the process whereby soot produced by an oxygen deficient flame is utilized are encumbered by: the small capability of producing only milligram quantities of fullerenes at most; loss of efficiency as the electrode diameter is increased; and the high expense-low yields of soot from benzene (about 0.5%) that result in C.sub.60 costs of at least about $100.00/g.
Accordingly, there is a need extant in the art of producing fullerenes to provide a method for producing fullerenes that is greater than the milligram quantities presently available through current technology, by providing higher percentages of soot containing higher percentages of fullerenes, at lower cost.